Frank details the pros and cons of several different shoulder holster systems in this post, which does an excellent job of discussion the various retention and safety issues inherent in shoulder rigs.
For my part, I’m not a fan of shoulder rigs – because I don’t work around machinery, a hip holster isn’t a problem for me in regards to catching on “stuff”. But the biggest reason I’m not a fan of the shoulder holster is that I like my “street” gear to replicate my competition gear as closely as possible. If I’ve spent hours anchoring my drawstroke to a hip carried firearm in competition, then if the balloon goes up, my hand is going to have a natural stress response to head for where it “thinks” my blaster should be.
If you’re a regular reader, you know the next thing I’m going to say: practice. If you decide that a shoulder holster system is your “cup of tea” you need to practice with it, while wearing “street” clothes. That’s complicated by the fact that a lot of ranges and every competition frowns on shoulder holsters, for the same safety reasons that Frank mentions. Luckily, you can always, always, use dry fire practice. Just remove all ammo from the room, double check that your gun is unloaded, and you’re off to the races.